<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      body {
        background-color: #000;
        margin: 0;
        overflow: hidden;
        background-repeat: no-repeat;
      }
    </style>
  </head>
  <body>
    <canvas id="canvas" width="1400" height="600"></canvas>
  </body>
</html>
<script>
  var canvas = document.getElementById("canvas");

  canvas.width = window.innerWidth;
  canvas.height = window.innerHeight;

  // Initialize the GL context
  var gl = canvas.getContext("webgl");
  if (!gl) {
    console.error("Unable to initialize WebGL.");
  }

  //Time
  var time = 0.0;

  //************** Shader sources **************

  var vertexSource = `
attribute vec2 position;
void main() {
  gl_Position = vec4(position, 0.0, 1.0);
}
`;

  var fragmentSource = `
precision highp float;

uniform float width;
uniform float height;
vec2 resolution = vec2(width, height);

uniform float time;

#define POINT_COUNT 8

vec2 points[POINT_COUNT];
const float speed = -0.5;
const float len = 0.25;
float intensity = 1.3;
float radius = 0.008;

//https://www.shadertoy.com/view/MlKcDD
//Signed distance to a quadratic bezier
float sdBezier(vec2 pos, vec2 A, vec2 B, vec2 C){
  vec2 a = B - A;
  vec2 b = A - 2.0*B + C;
  vec2 c = a * 2.0;
  vec2 d = A - pos;

  float kk = 1.0 / dot(b,b);
  float kx = kk * dot(a,b);
  float ky = kk * (2.0*dot(a,a)+dot(d,b)) / 3.0;
  float kz = kk * dot(d,a);

  float res = 0.0;

  float p = ky - kx*kx;
  float p3 = p*p*p;
  float q = kx*(2.0*kx*kx - 3.0*ky) + kz;
  float h = q*q + 4.0*p3;

  if(h >= 0.0){
    h = sqrt(h);
    vec2 x = (vec2(h, -h) - q) / 2.0;
    vec2 uv = sign(x)*pow(abs(x), vec2(1.0/3.0));
    float t = uv.x + uv.y - kx;
    t = clamp( t, 0.0, 1.0 );

    // 1 root
    vec2 qos = d + (c + b*t)*t;
    res = length(qos);
  }else{
    float z = sqrt(-p);
    float v = acos( q/(p*z*2.0) ) / 3.0;
    float m = cos(v);
    float n = sin(v)*1.732050808;
    vec3 t = vec3(m + m, -n - m, n - m) * z - kx;
    t = clamp( t, 0.0, 1.0 );

    // 3 roots
    vec2 qos = d + (c + b*t.x)*t.x;
    float dis = dot(qos,qos);

    res = dis;

    qos = d + (c + b*t.y)*t.y;
    dis = dot(qos,qos);
    res = min(res,dis);

    qos = d + (c + b*t.z)*t.z;
    dis = dot(qos,qos);
    res = min(res,dis);

    res = sqrt( res );
  }

  return res;
}


//http://mathworld.wolfram.com/HeartCurve.html
vec2 getHeartPosition(float t){
  return vec2(16.0 * sin(t) * sin(t) * sin(t),
              -(13.0 * cos(t) - 5.0 * cos(2.0*t)
              - 2.0 * cos(3.0*t) - cos(4.0*t)));
}

//https://www.shadertoy.com/view/3s3GDn
float getGlow(float dist, float radius, float intensity){
  return pow(radius/dist, intensity);
}

float getSegment(float t, vec2 pos, float offset, float scale){
  for(int i = 0; i < POINT_COUNT; i++){
    points[i] = getHeartPosition(offset + float(i)*len + fract(speed * t) * 6.28);
  }

  vec2 c = (points[0] + points[1]) / 2.0;
  vec2 c_prev;
  float dist = 10000.0;

  for(int i = 0; i < POINT_COUNT-1; i++){
    //https://tinyurl.com/y2htbwkm
    c_prev = c;
    c = (points[i] + points[i+1]) / 2.0;
    dist = min(dist, sdBezier(pos, scale * c_prev, scale * points[i], scale * c));
  }
  return max(0.0, dist);
}

void main(){
  vec2 uv = gl_FragCoord.xy/resolution.xy;
  float widthHeightRatio = resolution.x/resolution.y;
  vec2 centre = vec2(0.5, 0.5);
  vec2 pos = centre - uv;
  pos.y /= widthHeightRatio;
  //Shift upwards to centre heart
  pos.y += 0.02;
  float scale = 0.000015 * height;

  float t = time;

  //Get first segment
  float dist = getSegment(t, pos, 0.0, scale);
  float glow = getGlow(dist, radius, intensity);

  vec3 col = vec3(0.0);

  //White core
  col += 10.0*vec3(smoothstep(0.003, 0.001, dist));
  //Pink glow
  col += glow * vec3(1.0,0.05,0.3);

  //Get second segment
  dist = getSegment(t, pos, 3.4, scale);
  glow = getGlow(dist, radius, intensity);

  //White core
  col += 10.0*vec3(smoothstep(0.003, 0.001, dist));
  //Blue glow
  col += glow * vec3(0.1,0.4,1.0);

  //Tone mapping
  col = 1.0 - exp(-col);

  //Gamma
  col = pow(col, vec3(0.4545));

  //Output to screen
  gl_FragColor = vec4(col,1.0);
}
`;

  //************** Utility functions **************

  window.addEventListener("resize", onWindowResize, false);

  function onWindowResize() {
    canvas.width = window.innerWidth;
    canvas.height = window.innerHeight;
    gl.viewport(0, 0, canvas.width, canvas.height);
    gl.uniform1f(widthHandle, window.innerWidth);
    gl.uniform1f(heightHandle, window.innerHeight);
  }

  //Compile shader and combine with source
  function compileShader(shaderSource, shaderType) {
    var shader = gl.createShader(shaderType);
    gl.shaderSource(shader, shaderSource);
    gl.compileShader(shader);
    if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
      throw "Shader compile failed with: " + gl.getShaderInfoLog(shader);
    }
    return shader;
  }

  //From https://codepen.io/jlfwong/pen/GqmroZ
  //Utility to complain loudly if we fail to find the attribute/uniform
  function getAttribLocation(program, name) {
    var attributeLocation = gl.getAttribLocation(program, name);
    if (attributeLocation === -1) {
      throw "Cannot find attribute " + name + ".";
    }
    return attributeLocation;
  }

  function getUniformLocation(program, name) {
    var attributeLocation = gl.getUniformLocation(program, name);
    if (attributeLocation === -1) {
      throw "Cannot find uniform " + name + ".";
    }
    return attributeLocation;
  }

  //************** Create shaders **************

  //Create vertex and fragment shaders
  var vertexShader = compileShader(vertexSource, gl.VERTEX_SHADER);
  var fragmentShader = compileShader(fragmentSource, gl.FRAGMENT_SHADER);

  //Create shader programs
  var program = gl.createProgram();
  gl.attachShader(program, vertexShader);
  gl.attachShader(program, fragmentShader);
  gl.linkProgram(program);

  gl.useProgram(program);

  //Set up rectangle covering entire canvas
  var vertexData = new Float32Array([
    -1.0,
    1.0, // top left
    -1.0,
    -1.0, // bottom left
    1.0,
    1.0, // top right
    1.0,
    -1.0, // bottom right
  ]);

  //Create vertex buffer
  var vertexDataBuffer = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, vertexDataBuffer);
  gl.bufferData(gl.ARRAY_BUFFER, vertexData, gl.STATIC_DRAW);

  // Layout of our data in the vertex buffer
  var positionHandle = getAttribLocation(program, "position");

  gl.enableVertexAttribArray(positionHandle);
  gl.vertexAttribPointer(
    positionHandle,
    2, // position is a vec2 (2 values per component)
    gl.FLOAT, // each component is a float
    false, // don't normalize values
    2 * 4, // two 4 byte float components per vertex (32 bit float is 4 bytes)
    0 // how many bytes inside the buffer to start from
  );

  //Set uniform handle
  var timeHandle = getUniformLocation(program, "time");
  var widthHandle = getUniformLocation(program, "width");
  var heightHandle = getUniformLocation(program, "height");

  gl.uniform1f(widthHandle, window.innerWidth);
  gl.uniform1f(heightHandle, window.innerHeight);

  var lastFrame = Date.now();
  var thisFrame;

  function draw() {
    //Update time
    thisFrame = Date.now();
    time += (thisFrame - lastFrame) / 1000;
    lastFrame = thisFrame;

    //Send uniforms to program
    gl.uniform1f(timeHandle, time);
    //Draw a triangle strip connecting vertices 0-4
    gl.drawArrays(gl.TRIANGLE_STRIP, 0, 4);

    requestAnimationFrame(draw);
  }

  draw();
</script>
